A radioactive nucleus has a half-life of 5 x 108 years. Assuming that a sample of rock (say, in an asteroid) solidified right after the solar system formed, approximately what fraction of the radioactive element should be left in the rock today?

So the present mass will be 0.0017002 times of the initial mass of the sample.

Explanation:

Given:

Let the initial quantity on the formation of the solar system be,

Then the final quantity at present be:

So the present mass will be 0.0017002 times of the initial mass of the sample.

+15-35=-20

going in opposite directions

electrical potential energy =0.072 j

explanation:

electrical potential energy is given by

u= q δv

q= charge=6 c

δv= potential difference and is given by

δv= e d

e= electric field

d= distance=5 mm=0.005 m

δv=2.4 (0.005)=0.012 v

now electric potential energy= u= q δv

u=6 (0.012)

u=0.072 j